Fuel cells, which have the possibility of attaining an energy density 10 times as high as that of lithium ion secondary batteries and can be carried anywhere provided that a fuel is carried with it, are expected to bring about great changes in criteria for designing mobile machines and tools. In particular, direct methanol fuel cells (DMFCs), which use methanol as the fuel, are expected to be rendered small-sized and lightweight and become low-cost. Further, much attention is also focused on such characteristics thereof that, in spite of their being small-sized, they are excellent in startability, responsiveness to load, and stability as power sources capable of satisfying those long-period drive requirements imposed by portable apparatus, typically cellular phones and notebook computers, and can generate electricity so long as a fuel is fed thereto, hence can be used for a long period of time.
Currently, in developing fuel cells, in particular DMFC-type fuel cells, it is demanded that a highly active material to serve as an oxidation catalyst for electrodes be developed.
Pt-containing nanoparticles are known to show potent oxidative catalytic activity against hydrogen and methanol and are utilized as catalysts for typical polymer electrolyte fuel cell (PEFC) electrodes. Since such noble metal-based catalysts are expensive, the usage thereof is required to be as low as possible. Therefore, it is necessary to increase their catalytic activity. For producing highly active catalysts, however, it is necessary to render the catalyst metal surface area as large as possible and, therefore, it is required that catalyst nanoparticles with a particle diameter of about 2-3 nm as supported on carbon as a carrier or support be used as catalysts for PEFC electrodes.
For preparing such catalysts for PEFC electrodes as mentioned above, two methods are available; the method comprising reducing a metal in ionic form in a solution in the presence of carbon as a support to cause precipitation of catalyst nanoparticles on the support carbon (e.g. Physica B, Vol. 323, page 124 (2002) [Non-Patent Document 1]) and the method comprising causing catalyst nanoparticles in a colloidal solution to be adsorbed on carbon as a support (e.g. Nano Letters, Vol. 2, page 235 (2002) [Non-Patent Document 2]). The catalysts prepared by the above methods are generally subjected to heat treatment prior to use to thereby remove the organic matter remaining on the particle surface and thus expose the catalyst metal on the catalyst nanoparticle surface.
However, catalyst nanoparticles with a particle diameter of 2-3 nm have a very large surface energy and show dispersion instability. Therefore, as the period of use of the catalyst is prolonged, a problem arises, namely the catalyst nanoparticles aggregate/agglomerate together and accordingly the catalyst metal surface area decreases, resulting in decreases in catalytic activity.
As reported in Physica B, Vol. 323, page 124 [Non-Patent Document 1], the above problem can be solved by using carbon nanohorns as the support carbon and thereby enabling Pt nanoparticles with a particle diameter of about 2 nm to be dispersed without aggregation/fusion together. Further, in Japanese Kokai Publication No. 2002-134123 [Patent Document 1], a technology is disclosed which comprises forming a covering layer containing a reducing silicon-containing macromolecular compound on the surface of a support carbon powder and causing Pt nanoparticles to precipitate/be carried in the coating layer to thereby prevent Pt nanoparticles from aggregation. However, on the occasion of long-term use of PEFC electrodes, the transfer of catalyst nanoparticles on the support carbon cannot be avoided, and the catalyst nanoparticles that have been transferred aggregate/agglomerate together, possibly resulting in decreases in catalyst metal surface area and decreases in catalytic activity.
On the other hand, a technology of preparing metal nanoparticles having a porous inorganic oxide on the surface thereof is described, for example, in Langmuir, Vol. 12, page 4329 (1996) [Non-Patent Document 3]. The authors of this article report that Au nanoparticles can be coated with a porous SiO2 layer. However, there is no report as yet about a technology of preparing Pt-containing nanoparticles with a porous inorganic oxide on the surface thereof.
Patent Document 1: Japanese Kokai (Laid-Open) Publication No. 2002-134123    Non-Patent Document 1: Physica B, Vol. 323, pp. 124-126 (2002)    Non-Patent Document 2: Nano Letters, Vol. 2 (No. 3), pp. 235-240 (2002)    Non-Patent Document 3: Langmuir, Vol. 12 (No. 18), pp. 4329-4335 (1996)